The present invention relates to a method of manufacturing a metallic press die (punch and/or matrix) having an edge portion or high pressure portion for processing a workpiece therewith.
Among metallic press dies, in the case of a metallic trimming press die for trimming, for example, a plate-like workpiece into a predetermined shape, there is provided in a predetermined position in the metallic trimming press die a trimming blade in the form of an edge portion. This trimming blade must be harder than the remaining portion of the metallic trimming press die out of consideration of durabilities. Therefore, it has been normal practice to first form a predetermined portion of a base material of the metallic trimming press die into the shape of a trimming blade, and then to subject the formed portion to quenching by a heat treatment such as flame hardening, whereby a trimming blade of higher hardness is obtained. However, all materials for the metallic trimming press die are not always suitable for this kind of heat treatment and therefore there used to be possibilities that a desired hardness is not obtained even after heat treatment such as flame hardening or the like, or that quench cracks occur. As a solution, the following method is being employed. Namely, that portion of the base material for the metallic press die in which the trimming blade is formed is removed and a welding material for hard facing such as is specified in the Japanese Industrial Standards (JIS) Z3251 or the like is overlaid or clad by welding. Thereafter the portion thus overlaid is machined into a shape of a trimming blade to thereby form a trimming blade.
In the above-described method of overlaying the welding material for hard facing, the composition of the welding material is adjusted so as to obtain a desired hardness even without heat treatment after the overlaying welding. Therefore, it is difficult to perform machining, especially such as is accompanied by intermittent cutting work like milling, after the overlaying welding. It has therefore a disadvantage in that the machining work must include grinding work, with the result that the working steps increase. In other words, the material that can be subjected to intermittent cutting work is limited to material hardness of up to HRC (Rockwell C hardness) 45. The hardness after welding of the above-described welding material becomes HRC 50 or more; it must therefore be resorted to grinding work.
It may be possible to once anneal the overlaid portion to thereby decrease the hardness for machining purpose and thereafter to restore it, after the machining work, to a predetermined hardness by quench. This method, however, not only increases the heat-treating steps but also has a possibility of distorting a base material for metallic pressing die due to heat treatment. As a consequence, after the quench hardening another machining for correction such as grinding or the like must be performed, with the result that the machining steps cannot be reduced.
Furthermore, in case where the base material for the metallic pressing die is cast iron, the material is subjected to the influence of carbon which is contained therein in a large quantity. The hardness of the overlaid deposited metal therefore increases and the toughness decreases with consequent occurrence of cracks. In order to cope with such a problem, an underlaying of nickel, iron-nickel or the like is applied in the first layer to avoid the occurrence of cracks. However, because of the decrease in hardness of the deposited metal due to nickel, multi-layer overlaying becomes necessary.
The above-described problem also arises when an edge portion of a punch in a bend metallic press die is formed by the above-described overlaying welding method or when a high pressure portion of the die such as a bead portion of a die face or a shoulder portion of a punch of a draw metallic press die, is formed by the overlaying welding method.
Therefore, the present invention has an object of providing a method of manufacturing a metallic press die in which the machining after the overlaying welding is easy in forming in the metallic press die an edge portion or high pressure portion of the die for processing a workpiece therewith and in which, without the necessity of corrective machining, the edge portion or high pressure portion having a hardness above a predetermined value can be obtained.
According to the present invention, the foregoing and other objects are attained by a method of manufacturing a metallic press die having an edge portion or high pressure portion for processing a workpiece therewith, the method comprising the steps of overlaying a welding material to that portion of a base material for the metallic press die which forms the edge portion or high pressure portion, the welding material having a hardness after welding of HRC 45 or below; machining an overlaid portion formed by the preceding step into a predetermined shape of the edge portion or high pressure portion; and subjecting, after the machining step, the overlaid portion to a sub-zero treatment to increase a hardness of the overlaid portion.
As the welding material, there can be used a material which contains, as a basic composition thereof, 0.5-1.5% by weight of carbon (C), 0.2-2.0% by weight of silicon (Si), 0.3-6.0% by weight of manganese (Mn), 0.3-10.0% by weight of chromium (Cr), 0.3-10.0% by weight of cobalt (Co), and the remaining parts of iron (Fe) inclusive of unavoidable impurities, and a starting temperature of martensitic transformation of which is 150.degree. C. or below, or a material which contains, aside from the above-described basic composition, one or more of vanadium (V), nickel (N), molybdenum (Mo), tungsten (W), aluminum (Al) and copper (Cu). The welding material may be used in any of the forms of a coated or covered electrode for arc welding, welding wire and welding powder.
In the foregoing cases, the starting temperature of martensitic transformation (hereinbelow referred to as "Ms temperature") is defined to be the value calculated by the following formula, in which the value of each element is represented in % by weight. ##EQU1## The finishing temperature of martensitic transformation (hereinbelow referred to as "Mf temperature") is defined to be the value calculated by the following formula. EQU Mf(.degree. C.)=Ms-230
The reason for having set or defined the composition of the welding material to the above-described ranges is as follows. Namely, as regards carbon, since it is a most effective element in lowering the Ms temperature, larger quantities of other elements which lower the Ms temperature must be added if the carbon content is less than 0.5%, resulting in bad economy. If the carbon content exceeds 1.5%, on the other hand, the Ms temperature will become too low, with the result that other elements cannot be added any more. This brings about a problem in that the characteristics of toughness or the like are lowered. Further, as regards silicon, although it is an element which improves both the deoxidation effect and the flowability of welding material during welding, the effect cannot be attained if the content is less than 0.2% and, if the content exceeds 2.0%, on the contrary, the flowability during welding becomes so good that it becomes difficult to effect overlaying or to attain sufficient layers of cladding. As regards manganese, it is superior in deoxidation effect and in an effect of improving the toughness and is also superior next to carbon in lowering the Ms temperature. However, there will occur blow holes due to insufficient deoxidation if its content is less than 0.3% and if its content exceeds 6.0%, on the other hand, scales will be generated in so much quantity as to impair the workability. As regards cobalt, it has effects of retarding the precipitation of carbides and of decreasing the hardness after the overlaying welding. However, if its content is less than 0.3% the above-described effects cannot be obtained and, on the contrary, if its content exceeds 100% the Ms temperature increases too much and, in view of the fact that cobalt is an expensive element, the cost becomes too high. As regards chromium, it has a strong affinity with carbon so as to become a carbide of high hardness and therefore improves the abrasion resistance of the overlaid portion. However, if its content is less than 0.3%, such effects cannot be obtained and if its content exceeds 10.0%, on the other hand, the hardness becomes so high that the anti-cracking characteristics and toughness are impaired.
Aside from the above-described elements, other elements may be contained within such a limit as to keep the Ms temperature as calculated by the above-described formula at 150.degree. C. or below. As regards the elements such as Ti and Zr, though they have no direct relationship with the Ms temperature and the Mf temperature, they have effects of improving the deoxidation and workability or the like. Therefore, it is also acceptable to add these elements. Further, as regards impurities, aside from those elements which are absolutely necessary in making an alloy of this welding material, the presence of impurities will give rise to no particular problem as long as the Ms temperature is kept at 150.degree. C. or below.
When overlaying is performed on a cast iron base material or unworked material for metallic press die using the above welding material, the Ms temperature in the first and the second layers, in case of three-layer overlaying, will become the normal temperature or below due to the inclusion of carbon in the base material of the metallic press die. As a result, the structure of the deposited metal will mostly become untransformed austenite. Therefore, the hardness of the deposited metal is low to form a metal of relatively soft and tough, with the result that the machining thereof can be easily performed. In the third layer which is overlaid with the same welding material, martensitic transformation is not completed because the Ms temperature becomes 150.degree. C. or below due to the above-described particular composition of the welding material. The hardness as welded therefore becomes HRC 45 or below and the deposited material can also be easily machined. After the above-described overlaid portion has been machined, the overlaid portion is supercooled down to 0.degree. C. or below, in other words, is subjected to a sub-zero treatment using Dry Ice, liquefied nitrogen, freezer or the like. In such a treatment, martensitic transformation of the deposited metal begins to start and residual or persisting austenite is transformed into martensite. As a result, the hardness increases to HRC 45 or more and a deposited metal having a high resistance to abrasion can be obtained. Further, since the hardness is thus increased by supercooling, there will be no such problem, which is likely to happen with the conventional quench hardening, that the metal is oxidized or that a distortion is generated accompanied by the heat treatment of heating to an elevated temperature and cooling; no particular finishing process or the like is either required. Furthermore, even in case where the base material for the metallic press die is cast iron, there will be no such thing that the hardness of the overlaid portion is increased and the toughness decreased with consequent cracks under the influence of carbon which is contained in a large quantity in cast iron. Therefore, unlike the conventional method, there is no necessity of applying an underlaying of nickel, iron-nickel or the like to the first layer, and the working steps can thus be largely reduced.